Method for purification of phosphoric acid high purity polyphosphoric acid

ABSTRACT

The present invention provides a purification method of phosphoric acid, which includes bringing phosphoric acid containing arsenic into contact with hydrogen halide, thereby to remove the arsenic from the phosphoric acid, and a purification method of phosphoric acid, which includes bringing phosphoric acid containing arsenic into contact with hydrogen halide in the presence of a compound capable of generating hydrogen halide under acidic conditions. According to the method for purifying phosphoric acid of the present invention, a completely new method for removing arsenic in phosphoric acid is provided, which does not require a filtration or deaeration step or shows a great amount of residual sodium, unlike a sulfide coagulation method. This method is applicable to high concentration phosphoric acid, particularly polyphosphoric acid, and can be practiced at a low cost. In addition, a conventionally nonexistent high purity polyphosphoric acid having an arsenic (As) content of not more than 1 ppm and low contents of heavy metal, silica, sodium and the like can be provided.

TECHNICAL FIELD

The present invention relates to a method for purifying phosphoric acid,which can efficiently reduce the content of harmful heavy metal,particularly that of arsenic, and to high purity polyphosphoric acidobtained thereby, which is expected to be applicable to a broader rangeof use in the fields of food, medicaments and electronic materials.

BACKGROUND ART

As a production method of phosphoric acid, there have been known a wetmethod and a dry method, and phosphoric acid is industrially produced byone of these methods. In the wet method, phosphorous ore is dissolved insulfuric acid, a gypsum component is filtered off to produce dilutephosphoric acid having a low concentration, and the dilute phosphoricacid is concentrated to a desired level to give phosphoric acid. In thedry method, phosphorous ore is reduced in an electric furnace to giveyellow phosphorus, which is burnt to give phosphoric anhydride, which isthen hydrated to give phosphoric acid. In general, the wet method doesnot require reduction or combustion and is considered to be advantageousfrom the aspect of production cost. The dry method is considered to beadvantageous from the aspect of quality, because the production viaphosphoric anhydride reduces contamination of impurities derived fromphosphorous ore. In any production method, however, the obtainedphosphoric acid contains arsenic derived from phosphorous ore orphosphoric anhydride, which is harmful for human body, in generallyabout several dozen ppm, and the arsenic is removed by a sulfidecoagulation method according to the use of the phosphoric acid. Aconventional method for removing arsenic is applicable only tophosphoric acid having a P₂O₅ concentration of up to about 60%.

High concentration phosphoric acid, particularly polyphosphoric acid(P₂O₅ concentration of not less than 72.4%), is also produced by theaforementioned wet method or dry method, like phosphoric acid.Polyphosphoric acid produced by the dry method generally shows lowerheavy metal, silica and sodium contents because the production goesthrough phosphoric anhydride. However, it contains arsenic derived fromphosphoric anhydride in about 5-100 ppm, because a conventional methodfor removing arsenic cannot be applied to high concentration phosphoricacid. Polyphosphoric acid produced by the wet method shows a low arseniccontent of less than 1 ppm because a conventional method for removingarsenic can be applied before concentration. On the other hand, itcharacteristically shows high contents of heavy metal, silica and sodiumderived from phosphorous ore. High concentration phosphoric acid,particularly polyphosphoric acid, which characteristically shows a lowarsenic content and low contents of heavy metal, silica and sodium, hasnot been obtained.

When phosphoric acid having a P₂O₅ concentration of up to about 60%,which is obtained by a dry method, is subjected to a dearsenic treatmentby a conventional sulfide coagulation method and the like andconcentrated, polyphosphoric acid having low contents of arsenic, heavymetal, silica and sodium is presumably obtained. However, suchproduction method has not been put to practical use. This is becauseequipment for burning and hydrating yellow phosphorus and for highlyconcentrating the resulting phosphoric acid is required, which isextremely disadvantageous from the economical point of view, due to highfacility costs and high operation costs.

The arsenic contained in phosphoric acid and polyphosphoric acid isparticularly problematic for a use that requires high purity, in thefields of food, medicaments, electronic materials and the like. Giventhe high attention to the environmental problems in recent years,moreover, phosphoric acid and polyphosphoric acid having a low arseniccontent are desired as industrial phosphoric acid to be used for metalsurface treatment, dye processing and the like. Therefore, a method forremoving arsenic, which affords efficient operation at lower costs, isdemanded.

Known methods for removing arsenic from phosphoric acid include (1)sulfide coagulation method, (2) solvent extraction method and (3) ionexchange method. Of these, the sulfide coagulation method is mostpopular because the required steps and equipment are rather simple andthis method is operable at a low cost. This method includes bringingphosphoric acid into contact with hydrogen sulfide, or sodium sulfide,sodium hydrosulfide and the like, which produce hydrogen sulfide upondissolution in phosphoric acid, to allow precipitation of arsenic asarsenic sulfide, which is followed by separation for removal. Incontrast, the solvent extraction method and ion exchange method are lesspopular because of the complicated steps and complicated equipment, aswell as higher costs.

However, the sulfide coagulation method, too, requires filtrationequipment, such as vacuum filter, press filter, centrifuge separator andthe like. The precipitated arsenic sulfide tends to become a gel, whichmakes it difficult to separate and remove arsenic from phosphoric acid.For facilitated separation, a method (Japanese Unexamined PublicationNo. 6-48712) comprising passage through an activated carbon tower, and amethod (Japanese Unexamined Publication No. 6-100307) comprisingaddition of a chelating resin are used. These methods are unpreferablebecause the facility and treatment costs become higher.

When hydrogen sulfide remains in phosphoric acid, moreover, corrosion ofphosphoric acid is enhanced, thus necessitating deaeration of excesshydrogen sulfide with air or nitrogen, which in turn makes the stepcomplicated. When compounds such as sodium sulfide, sodium hydrosulfideand the like are used, this method cannot be applied to a use where asodium content matters, since sodium remains in a large amount inphosphoric acid.

In the case of the sulfide coagulation method, when the phosphoric acidconcentration is high, particularly in the case of polyphosphoric acid,phosphoric acid itself comes to have a higher viscosity, thereby makingthe separation for removal of arsenic sulfide more difficult. Therefore,this method is applicable to phosphoric acid having a P₂O₅ concentrationof about 60% at most.

The method for removing arsenic from phosphoric acid having a higherconcentration is disclosed in Japanese Examined Publication No.53-128595 as the purification method of phosphoric acid by a wet method.According to this method, sodium chloride is added to phosphoric acidhaving a P₂O₅ concentration of up to about 82% to allow reaction ofarsenic in phosphoric acid with chlorine for separation and removal.Because this method involves the use of sodium chloride in 1000-2000ppm, sodium remains in a great amount in phosphoric acid after removingarsenic.

The conventional methods for removing arsenic require a filtration stepand a deaeration step, both of which are costly, and sodium remains in agreat amount in phosphoric acid. They cannot be applied to highconcentration phosphoric acid having a P₂O₅ concentration of not lessthan 60%, particularly polyphosphoric acid. In addition, there has notbeen obtained high concentration phosphoric acid, particularlypolyphosphoric acid, having lower contents of arsenic, heavy metal,silica and sodium, by a method operable at a low cost. Therefore, thefield of use of polyphosphoric acid has been limited to the field freeof a high purity requirement.

The present invention provides a completely new method for removingarsenic from phosphoric acid, which is operable at a low cost, whichrequires no filtration step or deaeration step but is free of a largeamount of residual sodium, unlike sulfide coagulation method, and whichis applicable to high concentration phosphoric acid, particularlypolyphosphoric acid. In addition, the present invention aims atobtaining high purity polyphosphoric acid having low contents ofarsenic, heavy metal, silica and sodium, which has not beenconventionally obtained.

DISCLOSURE OF THE INVENTION

The present inventors have conducted intensive studies in an attempt tosolve the above-mentioned problems and found that the arsenic content ofphosphoric acid drastically falls during an organic synthetic reactionaccompanying production of hydrogen chloride in high concentrationphosphoric acid. Further investigation thereof has revealed that thisorganic synthetic reaction itself has nothing to do with an arsenicremoval effect but, by bringing phosphoric acid into contact withhydrogen halide, the arsenic content of phosphoric acid can be reducedto not more than about 1 ppm, that, when the contact is performed in thepresence of a compound capable of generating hydrogen halide underacidic conditions, the arsenic removal effect is enhanced, and that thismethod is useful for removing arsenic particularly from polyphosphoricacid, which resulted in the completion of the present invention.

Accordingly, the present invention provides the following.

-   (1) A method for purifying phosphoric acid, which comprises bringing    phosphoric acid containing arsenic into contact with hydrogen    halide, thereby to remove the arsenic from the phosphoric acid.-   (2) The purification method of (1) above, wherein the phosphoric    acid containing the arsenic is brought into contact with hydrogen    halide in the presence of a compound capable of generating hydrogen    halide under acidic conditions.-   (3) The purification method of (2) above, wherein the compound    capable of generating hydrogen halide under acidic conditions is    added in a proportion of less than 1 wt % of a weight of the    phosphoric acid.-   (4) The purification method of (2) above, wherein the compound    capable of generating hydrogen halide under acidic conditions is a    chloride of iron(II), copper(I) or tin(II).-   (5) The purification method of (1) above, wherein the hydrogen    halide is hydrogen chloride.-   (6) The purification method of (1) above, wherein the phosphoric    acid has a P₂O₅ concentration of not less than 72.4%.-   (7) A high purity polyphosphoric acid having an iron (Fe) content of    not more than 20 ppm, a sodium (Na) content of not more than 100    ppm, a silica (SiO₂) content of not more than 50 ppm and an arsenic    (As) content of not more than 1 ppm.-   (8) The high purity polyphosphoric acid of (7) above, wherein the    iron (Fe) content is not more than 10 ppm, the sodium (Na) content    is not more than 5 ppm, the silica (SiO₂) content is not more than 5    ppm and the arsenic (As) content is not more than 1 ppm.-   (9) The high purity polyphosphoric acid of (7) above, which has a    chromium (Cr) content of not more than 5 ppm, a nickel (Ni) content    of not more than 5 ppm and a molybdenum (Mo) content of not more    than 5 ppm.-   (10) The high purity polyphosphoric acid of (8) above, wherein the    chromium (Cr) content is not more than 2 ppm, the nickel (Ni)    content is not more than 2 ppm and the molybdenum (Mo) content is    not more than 2 ppm.

Before explaining the present invention in detail, the indication of thephosphoric acid concentration is explained in the following.

In the present invention, phosphoric acid is a polycondensation productof orthophosphoric acid, and its concentration is generally expressedafter orthophosphoric acid conversion or P₂O₅ conversion. Phosphoricacid having an orthophosphoric acid concentration of 100% corresponds tophosphoric acid having a P₂O₅ concentration of 72.4%, wherein therelationship between both concentrations is expressed by the followingequation:P₂O₅ concentration (%)=orthophosphoric acid concentration (%)×0.724

Phosphoric acid having a P₂O₅ concentration of less than 72.4% is anaqueous phosphoric acid solution containing a polycondensation productof orthophosphoric acid and water in an equilibrium state. Phosphoricacid having a P₂O₅ concentration of not less than 72.4% consists solelyof a polycondensation product of orthophosphoric acid and is calledpolyphosphoric acid, strong phosphoric acid or super phosphoric acid. Inthe present invention, any phosphoric acid concentration is expressed inP₂O₅ concentration.

In a first embodiment of the method for purifying phosphoric acidaccording to the present invention, phosphoric acid containing arsenicis brought into contact with hydrogen halide alone and the arsenic isremoved from the phosphoric acid. The reason why an arsenic removaleffect is expressed by this method is uncertain, but it is postulatedthat hydrogen halide causes arsenic in phosphoric acid to convert to anarsenic compound having a high volatility, such as arsenic halide andarsenic hydride, and the compound evaporates at a treatment temperatureof the purification method of the present invention and is dischargedfrom the system along with hydrogen halide. It is also unknown as to thechemical state of arsenic in phosphoric acid, but the arsenic isconsidered to be in the state of arsenic acid, arsenious acid, diarsenicpentaoxide, diarsenic trioxide and the like.

While the phosphoric acid concentration applicable to the firstembodiment is free of any particular limitation, when the method isapplied to low concentration phosphoric acid, hydrogen halide isdissolved in water to enhance corrosion of phosphoric acid and to causehydrogen halide to remain after treatment. Thus, this method isadvantageously applied to phosphoric acid having relatively lower watercontent (high phosphoric acid concentration). Particularly, when it isapplied to polyphosphoric acid, which is completely anhydrous, hydrogenhalide does not remain in polyphosphoric acid, thus enabling eliminationof corrosion. When the phosphoric acid to be used is polyphosphoricacid, a concentration range free of freezing point, namely, a P₂O₅concentration of 75-77% or not less than 80% is preferable. Inconsideration of handling property, the upper limit of P₂O₅concentration is preferably 90%.

In a second embodiment of the method for purifying phosphoric acid ofthe present invention, phosphoric acid containing arsenic is broughtinto contact with hydrogen halide in the presence of a compound capableof generating hydrogen halide under acidic conditions, and the arsenicis removed from the phosphoric acid. In this embodiment, arsenic can beremoved highly efficiently. The compound capable of generating hydrogenhalide under acidic conditions may be an organic or inorganic compound.Particularly, it is preferably metal halide having a reducing action,more preferably halide of iron(II), copper(I) or tin(II), mostpreferably chloride of these metals, particularly preferably chloride ofiron(II) or tin(II).

When phosphoric acid containing arsenic is brought into contact withhydrogen halide in the presence of a compound capable of generatinghydrogen halide under acidic conditions, the arsenic removal effect ispromoted for unknown reasons, but it is postulated that arsenic itselfbecomes easily reducible and easily becomes a volatile arsenic compound,such as arsenic halide and arsenic hydride, which is easily evaporatedat the treatment temperature of the purification method of the presentinvention, because the effect is particularly striking when metal halideacting as a reducing agent is added.

The compound capable of generating hydrogen halide under acidicconditions in the second embodiment is added in a trace amount, which ispreferably less than 1 wt %, more preferably less than 0.5 wt %, of aweight of phosphoric acid. When it is used in a greater amount, anarsenic removal effect is assumed to be enhanced more. Particularly whenthis compound is metal halide, however, an impurity derived from a metalin the metal halide unpreferably remains. When the residual impurity isto be avoided, or when high purity phosphoric acid, particularly highpurity polyphosphoric acid is desired, the first embodiment whereinphosphoric acid containing arsenic is brought into contact with hydrogenhalide alone is preferable rather than the second embodiment.

In the first and the second embodiments, phosphoric acid is brought intocontact with hydrogen halide in any typical gas-liquid mixing apparatus,which may be of a batch type or a continuous type. For example, a methodwherein a batch reaction vessel is used and phosphoric acid isvigorously stirred with direct blowing of hydrogen halide therein, amethod wherein a static mixer or ejector is used and phosphoric acid andhydrogen halide are continuously mixed, a method wherein the bothmethods are combined, and the like can be employed. The hydrogen halideto be used in the present invention may be hydrogen chloride, hydrogenbromide and the like, with preference given to hydrogen chloride becauseit is obtained most easily and economically.

In the first and the second embodiments, the necessary amount ofhydrogen halide relative to phosphoric acid depends greatly on thegas-liquid mixing efficiency of the apparatus used, and the range of theamount is difficult to specify. Because arsenic in phosphoric acid isconsidered to be trivalent or pentavalent, at least not less than 3 to 5molar equivalents of hydrogen halide relative to the arsenic content isconsidered to be necessary. In the case of the easiest method wherein abatch reaction vessel is used and phosphoric acid is vigorously stirredwith direct blowing of hydrogen halide therein, for example, hydrogenchloride is blown into 500 ml of polyphosphoric acid (P₂O₅ concentration85%) having an arsenic content of 50 ppm preferably at a rate of 1-100ml/min, more preferably 10-80 ml/min, for preferably not less than 30min, more preferably not less than 60 min.

In the first and the second embodiments, the treatment temperaturevaries depending on the phosphoric acid concentration and the materialof the reactor, but may be any within the range of from about 50° C. to200° C. In consideration of the corrosion of a stainless materialindustrially used for a reactor and the like by phosphoric acid, it isdesirably not more than 150° C. For promotion of evaporation of thegenerated arsenic compound, it is desirably at least 50° C., morepreferably not less than 100° C. The pressure is free of any particularlimitation and can be set optionally, wherein a sufficient dearseniceffect can be achieved under an atmospheric pressure.

In the first and the second embodiments, moreover, the arsenic compoundis discharged from the system together with hydrogen halide. Thesecompounds may be preferably absorbed by water and neutralized, orabsorbed by an aqueous alkaline solution of sodium hydroxide, potassiumhydroxide and the like.

According to the method for purifying phosphoric acid of the presentinvention, phosphoric acid having a low arsenic content of not more thanabout 1 ppm can be obtained by an easy method. Particularly, when themethod is applied in the presence of a compound capable of generatinghydrogen halide under acidic conditions, phosphoric acid having a lowarsenic content of not more than about 0.1 ppm can be obtained. In asmuch as the method does not require an equipment cost for a filtrationstep or a deaeration step or the use of a sodium compound, such assodium sulfide, sodium hydrosulfide and the like, unlike theconventional method, it does not happen that sodium remains in a largeamount in phosphoric acid. By the application of the purification methodof the present invention to phosphoric acid having low contents of heavymetal, sodium and silica, which is obtained by the dry method,phosphoric acid having a low arsenic content and low contents of heavymetal, silica and sodium can be obtained.

The method for purifying phosphoric acid of the present invention doesnot require conventional filtration steps. Therefore, the method isapplicable to high concentration phosphoric acid, particularlypolyphosphoric acid. By the application of the purification method ofthe present invention to polyphosphoric acid having low contents ofheavy metal, silica and sodium but a high arsenic content, which isobtained by the dry method, high purity polyphosphoric acid having a lowarsenic content and low contents of heavy metal, silica and sodium,which has not been obtained heretofore, can be afforded.

To be specific, by the application of the purification method of thepresent invention to polyphosphoric acid prepared by adding phosphoricanhydride to industrial phosphoric acid and dissolving therein,polyphosphoric acid having an iron (Fe) content of not more than 20 ppm,a sodium (Na) content of not more than 100 ppm, a silica (SiO₂) contentof not more than 50 ppm, and an arsenic (As) content of not more than 1ppm, and further, a chromium (Cr) content of not more than 5 ppm, anickel (Ni) content of not more than 5 ppm, and a molybdenum (Mo)content of not more than 5 ppm can be obtained.

By the application of the purification method of the present inventionto polyphosphoric acid prepared by adding phosphoric anhydride tophosphoric acid as a reagent or phosphoric acid as a food additive anddissolving therein, polyphosphoric acid having an iron (Fe) content ofnot more than 10 ppm, a sodium (Na) content of not more than 5 ppm, asilica (SiO₂) content of not more than 5 ppm, and an arsenic (As)content of not more than 1 ppm, and further, a chromium (Cr) content ofnot more than 2 ppm, a nickel (Ni) content of not more than 2 ppm, and amolybdenum (Mo) content of not more than 2 ppm can be obtained.

The content of each element of the polyphosphoric acid in the presentinvention was measured by the following methods.

Arsenic was measured according to JIS-K 0102 (1993). In as much as anorganic substance is considered to be absent from a polyphosphoric acidsample, an organic substance decomposition step with sulfuricacid/nitric acid as described in the JIS was omitted. Iron, chromium,nickel, molybdenum and silica were quantitatively determined by aninductively coupled plasma (ICP) analysis. The ICP sample solution wasprepared by adding hydrochloric acid to polyphosphoric acid to give 1.2M hydrochloric acid solution. A calibration curve of the phosphoric acidsolution was adjusted to achieve the same concentration as the samplesolution. Sodium was quantitatively determined by atomic absorptionspectrometry. The atomic absorption sample solution was the same as itwas for ICP. The P₂O₅ concentration of polyphosphoric acid wasdetermined by titration using 1 M sodium hydroxide solution.

The present invention is explained in more detail in the following byreferring to Examples. The present invention is not limited by theseexamples.

EXAMPLE 1

Commercially available polyphosphoric acid obtained by a dry method (500ml, P₂O₅ concentration 84.9%) having an arsenic content of 58 ppm wasstirred with heating at 130° C., during which a glass tube having aglass bowl filter was inserted and hydrogen chloride gas was blown in atan aeration rate of 20 ml/min for about 2 hours to give high puritypolyphosphoric acid. The discharged hydrogen chloride gas was trapped inan aqueous sodium hydroxide solution. The analysis results of theobtained high purity polyphosphoric acid are shown in Table 1.

EXAMPLE 2

Commercially available polyphosphoric acid obtained by a dry method (500ml, P₂O₅ concentration 84.2%) having an arsenic content of 8 ppm wascast in a 1(one) liter three-necked glass flask and stirred with heatingat 150° C., during which a glass tube having a glass bowl filter wasinserted and hydrogen chloride gas was blown in at an aeration rate of50 ml/min for about 2 hours to give high purity polyphosphoric acid. Thedischarged hydrogen chloride gas was trapped in an aqueous sodiumhydroxide solution. The analysis results of the obtained high puritypolyphosphoric acid are shown in Table 1.

EXAMPLE 3

Phosphoric anhydride (200 g) having an arsenic content of 25 ppm wasadded to phosphoric acid as a reagent (422 g, P₂O₅ concentration 61.5%)having an arsenic content of 0.1 ppm. The mixture was stirred withheating at 120° C. to give a homogeneous liquid, whereby polyphosphoricacid (P₂O₅ concentration 84.0%) having an arsenic content of 8 ppm wasprepared. While stirring under heating at 150° C., a glass tube with aglass bowl filter was inserted and hydrogen chloride gas was blown in atan aeration rate of 20 ml/min for about 3 hours to give high puritypolyphosphoric acid. The discharged hydrogen chloride gas was trapped inan aqueous sodium hydroxide solution. The analysis results of theobtained high purity polyphosphoric acid are shown in Table 1.

EXAMPLE 4

Phosphoric anhydride (170 g) having an arsenic content of 25 ppm wasadded to commercially available polyphosphoric acid obtained by a drymethod (330 g, P₂O₅ concentration 76.0%) having an arsenic content of 5ppm. The mixture was stirred under heating at 120° C. to give ahomogeneous liquid, whereby polyphosphoric acid (P₂O₅ concentration84.2%) having an arsenic content of 11 ppm was prepared. While stirringunder heating at 150° C., a glass tube with a glass bowl filter wasinserted and hydrogen chloride gas was blown in at an aeration rate of20 ml/min for about 3 hours to give high purity polyphosphoric acid. Thedischarged hydrogen chloride gas was trapped in an aqueous sodiumhydroxide solution. The analysis results of the obtained high puritypolyphosphoric acid are shown in Table 1.

EXAMPLE 5

Industrial phosphoric acid (500 ml, P₂O₅ concentration 65%) having anarsenic content of 15 ppm was cast in a 1(one) liter three-necked glassflask and stirred with heating at 130° C., during which a glass tubehaving a glass bowl filter was inserted and hydrogen chloride gas wasblown in at an aeration rate of 50 ml/min. The discharged hydrogenchloride gas was trapped in an aqueous sodium hydroxide solution. As aresult, the concentration of arsenic in phosphoric acid at 180 minutesafter the start of blowing was 0.8 ppm.

EXAMPLE 6

The same treatment as in Example 5 was applied except that ironchloride(II) (0.1 g) was added before blowing in hydrogen chloride. As aresult, the concentration of arsenic in phosphoric acid at 120 minutesafter the start of blowing was 0.07 ppm.

EXAMPLE 7

The same treatment as in Example 5 was applied except that tinchloride(II) dihydrate (0.5 g) was added before blowing in hydrogenchloride. As a result, the concentration of arsenic in phosphoric acidat 120 minutes after the start of blowing was 0.08 ppm.

The analysis results of each element contained in the high puritypolyphosphoric acids obtained in Examples 1-4 are shown in Table 1. Forcomparison, the analysis results of commercially available product A,which is polyphosphoric acid obtained by a dry method, and commerciallyavailable product B, which is polyphosphoric acid obtained by a wetmethod, are also shown in Table 1.

TABLE 1 Ex- Ex- Commer- Commer- am- am- cially cially Analysis pleExample ple Example available available item 1 2 3 4 product A product BP₂O₅ con- 84.9 84.2 84.0 84.2 85.0 84.3 centra- tion (%) Fe 1.6 1.0 0.21.2 1.6 37 content Cr 0.8 0.1 0.1 0.7 0.8 6.3 content Ni 0.6 0.3 0.1 0.40.6 4.4 content Mo <0.5 <0.5 <0.5 <0.5 <0.5 12 content Na 0.1 0.1 0.10.1 0.2 250 content SiO₂ <5 <5 <5 <5 <5 120 content As 0.5 0.08 0.030.03 58 0.02 contentIn Table 1, respective contents are in the unit of ppm.

As is evident from Table 1, the commercially available product A(polyphosphoric acid obtained by a dry method) showed low contents ofheavy metal, silica and sodium but a high arsenic content, and thecommercially available product B (polyphosphoric acid obtained by a wetmethod) showed a low arsenic content but high contents of heavy metal,sodium and silica. In contrast, the high purity polyphosphoric acids ofExamples 1-4 showed low arsenic contents, and low contents of heavymetal, sodium and silica. From Examples 6 and 7, it is evident that,when phosphoric acid containing arsenic is brought into contact withhydrogen halide in the presence of a compound capable of generatinghydrogen halide under acidic conditions, such as iron chloride(II), tinchloride(II) and the like, an effect of removing arsenic in phosphoricacid can be enhanced.

As is evident from the following explanation, the method for purifyingphosphoric acid of the present invention can effectively remove arsenicin phosphoric acid, which is harmful for human being, by a simplemanipulation wherein phosphoric acid is brought into contact withhydrogen halide. To be specific, the arsenic content of several dozenppm of general industrial phosphoric acid can be reduced to not morethan about 1 ppm. When the above-mentioned step is applied in thepresence of a compound capable of generating hydrogen halide underacidic conditions, the removal to the level of not more than about 0.1ppm is possible.

This method includes a simple treatment step and requires no specialapparatus. As a result, the total cost for the treatment is low, andsince a sodium compound is not used, unlike the conventional method,sodium does not remain in a large amount in phosphoric acid.

Moreover, since the method for purifying phosphoric acid of the presentinvention does not require a filtration step, unlike a conventionalpurification method, it can be applied to high concentration phosphoricacid, particularly polyphosphoric acid. Particularly, whenpolyphosphoric acid obtained by a dry method and having low contents ofheavy metal, sodium and silica but a high arsenic content is subjectedto the purification method of the present invention, high puritypolyphosphoric acid having a low arsenic content and low contents ofheavy metal, silica and sodium can be afforded, which has not beenobtained heretofore. Such high purity polyphosphoric acid is highly safeand can reduce the environmental burden. Therefore, it is applicable toa broader range of use in the fields of food, medicaments and electronicmaterials.

This application is based on patent application Nos. 373696/1998 and230628/1999 filed in Japan, the contents of which are herebyincorporated by reference.

1. A method for purifying phosphoric acid, which comprises bringingphosphoric acid containing arsenic into contact with hydrogen halide inthe presence of a compound capable of generating hydrogen halide underacidic conditions, wherein the compound capable of generating hydrogenhalide under acidic conditions is a halide of iron (II), copper (I) ortin (II), thereby to remove the arsenic from the phosphoric acid.
 2. Thepurification method of claim 1, wherein the compound capable ofgenerating hydrogen halide under acidic conditions is added in aproportion of less than 1 wt % of a weight of the phosphoric acid. 3.The purification method of claim 1, wherein the compound capable ofgenerating hydrogen halide under acidic conditions is a chloride ofiron(II), copper(I) or tin(II).
 4. The purification method of claim 1,wherein the hydrogen halide is hydrogen chloride.
 5. The purificationmethod of claim 1, wherein the phosphoric acid has a P₂O₅ concentrationof not less than 72.4%.